Virtual image display device

ABSTRACT

A virtual image display device includes a projection optical system that includes a projection mirror that projects an image display light toward a concavely curved surface of a virtual image presentation plate. The projection mirror has a convexly curved surface on which the image display light is incident and reflected diagonally and is arranged such that a direction orthogonal to both a direction of incidence and a direction of reflection of the image display light on the concavely curved surface and a direction orthogonal to both a direction of incidence and a direction of reflection of the image display light on the convexly curved surface are parallel to each other. The display unit is provided at a focal point of a composite optical system within a meridional plane, the composite optical system being formed by the virtual image presentation plate and the projection optical system.

CROSS REFERENCE TO RELATED APPLICATION

Priority is claimed to Japanese Patent Application No. 2018-003138,filed on Jan. 12, 2018, the entire content of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a virtual image display device.

2. Description of the Related Art

Recently, head-up displays are available for use as display devices forvehicles. A head-up display projects an image display light toward, forexample, a windshield of a vehicle, superimposes a virtual image basedon the image display light on the scenery outside the vehicle, anddisplays the resultant image. A windshield has two interfaces inside andoutside the vehicle. The image display lights reflected at therespective interfaces and visually perceived may be superimposed with ashift and viewed as double images. To inhibit double images from beingproduced, there is proposed a mathematical expression for defining aviewing distance to contain the amount of shift between double imageswithin the resolution of the human eyes and obtaining an opticalarrangement that realizes the viewing distance.

SUMMARY OF THE INVENTION

The technology described above requires a large viewing distance to theposition of presentation of a virtual image ahead of the user in orderto reduce double images. It is preferred to reduce the occurrence ofdouble images suitably, regardless of the viewing distance to a virtualimage.

The embodiments address the above-described issue, and a general purposethereof is to provide a technology for improving the visibility of avirtual image presented.

An embodiment of the present invention relates to a virtual imagedisplay device for presenting a virtual image to a user via a virtualimage presentation plate, including: a display unit that generates animage display light; and a projection optical system that includes aprojection mirror that projects the image display light toward thevirtual image presentation plate such that the image display light isincident and reflected on a concavely curved surface of the virtualimage presentation plate diagonally. The projection mirror has aconvexly curved surface on which the image display light is incident andreflected diagonally and is arranged such that a direction orthogonal toboth a direction of incidence and a direction of reflection of the imagedisplay light on the concavely curved surface and a direction orthogonalto both a direction of incidence and a direction of reflection of theimage display light on the convexly curved surface are parallel to eachother. The display unit is provided at a focal point of a compositeoptical system within a meridional plane, the composite optical systembeing formed by the virtual image presentation plate and the projectionoptical system, and the focal point of the composite optical systemwithin the meridional plane is a position of convergence of parallelbeams assumed to be incident from the user toward the virtual imagepresentation plate along the meridional plane of the virtual imagepresentation plate.

Optional combinations of the aforementioned constituting elements, andmutual substitution of constituting elements and implementations of thepresent invention between methods, apparatuses, and systems may also bepracticed as additional modes of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of examples only, withreference to the accompanying drawings which are meant to be exemplary,not limiting and wherein like elements are numbered alike in severalFigures in which:

FIG. 1 schematically shows a configuration of a virtual image displaydevice according to the embodiment;

FIG. 2 schematically shows the occurrence of double images induced bythe virtual image presentation plate;

FIG. 3 schematically shows inhibition of double images effected by awedge glass;

FIG. 4 shows an optical arrangement in a virtual image display deviceaccording to the comparative embodiment in detail;

FIGS. 5A and 5B schematically show the astigmatism of parallel beamsincident on a partial region on a concavely curved surface;

FIGS. 6A, 6B, and 6C show a configuration of the virtual image displaydevice according to the first embodiment in detail;

FIG. 7 shows a configuration of a virtual image display device accordingto the second embodiment in detail;

FIG. 8 shows a configuration of a virtual image display device accordingto the third embodiment in detail;

FIGS. 9A and 9B show a configuration of a virtual image display deviceaccording to the fourth embodiment in detail; and

FIG. 10 shows a configuration of a virtual image display deviceaccording to the fifth embodiment in detail.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention.

A description will be given of embodiments of the present invention withreference to the drawings. Specific numerical values are shown in theembodiments by way of example only to facilitate the understanding ofthe invention and should not be construed as limiting the scope of theinvention unless specifically indicated as such. Those elements in thespecification and drawings that have substantially the identicalfunction and configuration are represented by the same referencesymbols, and the description is not duplicated. Elements not directlyrelevant to the invention are omitted from the illustration.

FIG. 1 schematically shows a configuration of a virtual image displaydevice 10 according to the embodiment. In this embodiment, the virtualimage display device 10 is installed in a dashboard of a vehicle 60,which exemplifies a moving object. The virtual image display device 10is a so-called head-up display device. The virtual image display device10 presents a virtual image 50 in front of the vehicle 60 in thedirection of travel (rightward direction in FIG. 1) by projecting animage display light toward a windshield 62, which is a virtual imagepresentation plate. The user E (e.g., the driver) can see the virtualimage 50 superimposed on the actual scenery via the windshield 62.Therefore, the driver E can access information shown in the virtualimage 50 substantially without moving the line of sight while drivingthe vehicle. Referring to the FIG. 1, the direction of travel of thevehicle 60 (longitudinal direction) is defined as the z direction, thevertical direction of the vehicle 60 (up/down direction) is defined asthe y direction, and the transversal direction of the vehicle 60 isdefined as the x direction.

The virtual image display device 10 includes an illumination unit 11, adisplay unit 12, a projection optical system 14, and a control unit 40.The illumination unit 11 is a light source for generating a displaylight and generates an illumination light for illuminating the displayunit 12. The illumination unit 11 includes a light emitting device suchas a light emitting diode (LED) and a laser diode (LD), and an opticaldevice for adjusting the light intensity distribution and angledistribution of the light output from the light emitting device. Theillumination unit 11 provides the display unit 12 with a substantiallyuniform white light. The configuration of the illumination unit 11 isnot limited to any particular type. For example, a light emitting devicesuch as a light tunnel, a Fresnel lens, and a light diffusion plate maybe used to condition the output light from the light emitting device.

The display unit 12 modulates an illumination light from theillumination unit 11 to generate a display light and forms anintermediate image (real image) corresponding to the display content ofthe virtual image 50. The display unit 12 includes an image displaydevice of transmission type for generating a display light. For example,the display unit 12 includes a display device such as a liquid crystalpanel of transmission type. The image display device receives an imagesignal transmitted from the control unit 40 and generates an imagedisplay light for the display content corresponding to the image signal.The display unit 12 may further include an optical device forconditioning the orientation and light distribution angle of the imagedisplay light. Further, the display unit 12 may be configured bycombining an element other than a liquid crystal panel of transmissiontype and a screen of transmission type such as a microlens array sheetand a light diffusion sheet. The element is exemplified by a projectionunit such as a digital micromirror device (DMD), a liquid crystal onsilicon (LOCS) laser scanning module (LSM), and a micro electromechanical system (MEMS) LSM.

The projection optical system 14 projects the image display lightgenerated by the display unit 12 toward the windshield 62. Theprojection optical system 14 includes an optical device of transmissiontype such as a convex lens and an optical device of reflection type suchas a concave mirror. The specific configuration of the projectionoptical system 14 will be described separately.

The control unit 40 generates an image for display and causes theillumination unit 11 and the display unit 12 to operate to present thevirtual image 50 corresponding to the image for display. The controlunit 40 is connected to an external device 64 and generates the imagefor display based on the information from the external device 64.

The external device 64 is a device for generating original data for animage displayed as the virtual image 50. For example, the externaldevice 64 may be an Electronic Control Unit (ECU) for the vehicle 60, anavigation device, or a mobile device such as a cell phone, smartphone,and tablet. The external device 64 transmits, to the control unit 40,image data necessary to display the virtual image 50, informationindicating the content and type of the image data, and informationrelated to the vehicle 60 such as the speed and current position of thevehicle 60.

Before describing the optical arrangement according to the embodiment indetail, a description will be given of the occurrence of double imageswith reference to a comparative example. One factor that causes thevirtual image 50 to be viewed as double images is that the image displaylights reflected at the two interfaces of the windshield 62 inside andoutside the vehicle and visually perceived are presented with a shift.

FIG. 2 schematically shows the occurrence of double images induced bythe virtual image presentation plate 22. In FIG. 2, an optical devicesuch as a concave mirror provided between the virtual image presentationplate 22 and a display unit 92 is omitted for simplify the description.The virtual image presentation plate 22 has a predetermined thickness tand has a first surface 23 and a second surface 24. The first surface 23corresponds to the interface of the windshield 62 inside the vehicle,and the second surface 24 corresponds to the interface of the windshield62 outside the vehicle.

The image display light arriving at the user E from an arbitrary point Eof the display unit 92 mainly travels on two light paths L1, L2. Thefirst light path L1 is a light path in which the light is reflected bythe first surface 23 and travels toward the user E. The second lightpath L2 is a light path in which the light is refracted by the firstsurface 23, reflected by the second surface 24, and then re-refracted bythe first surface 23 and travels toward the user E. If there is anangular difference Δθ between the first light path L1 and the secondlight path L2 leading toward the user E, the image display lightstraveling on the two light paths L1, L2 respectively are visuallyperceived as being shifted from each other according to the angulardifference Δθ, producing double images in a virtual image 51. While itis possible to assume a light path in which the light is reflectedmultiple times between the first surface 23 and the second surface 24and travels toward the user E, the component of the image display lightreflected multiple times and traveling toward the user E is small andcan be neglected in a normal mode of usage.

FIG. 3 schematically shows inhibition of double images effected by awedge glass. A virtual image presentation plate 82 shown in FIG. 3 is aso-called “wedge glass” and is configured such that the thickness of thevirtual image presentation plate 82 varies depending on the location.This results in a first surface 83 and a second surface 84 of thevirtual image presentation plate 82 having mutually different angles ofinclination relative to a direction of line of sight of the user E, andan angular difference δ is provided between the surfaces. By using awedge glass in which the angular difference δ is provided between thetwo surfaces 83, 84, the angular difference Δθ between the first lightpath L1 and the second light path L2 as shown in FIG. 2 is corrected,and a virtual image 52 in which double images are reduced is presented.

However, a “wedge glass” like this need be formed by controlling theangular difference δ with high precision and so is more expensive thanan ordinary glass having a uniform thickness t. Further, forming thewindshield 62 of the vehicle 60 by using a wedge glass not only requiresa dedicated wedge glass adapted to the shape of the vehicle 60 but alsorequires replacing the entirety of the windshield 62 so that a heavycost will be incurred. It is therefore preferred to reduce theoccurrence of double images without using a special wedge glass.

FIG. 4 shows an optical arrangement in a virtual image display device 90according to the comparative embodiment in detail. The comparativeexample differs from the configuration of FIG. 2 in that a convex lens94 is provided between the virtual image presentation plate 22 and thedisplay unit 92. Thus, according to the comparative example, the angulardifference between the first light path L1, in which the light leavingan arbitrary point on the display unit 92 and reflected by the firstsurface 23 of the virtual image presentation plate 22, and the secondlight path L2, in which the light is reflected by the second surface 24of the virtual image presentation plate 22, is reduced by providing theconvex lens 94. In particular, by providing the display unit 92 at thefocal point of the composite optical system formed by the virtual imagepresentation plate 22 and the convex lens 94, the angular differencebetween the first light path L1 and the second light path L2 is removedand double images are eliminated.

In the comparative example of FIG. 4, the image display light L iscaused to be diagonally incident on the virtual image presentation plate22 so that astigmatism could be produced if the virtual imagepresentation plate 22 includes a curved surface. The windshield 62 ofautomobiles in general includes a curved surface and is configured suchthat the first surface 23 is concavely curved so that diagonal incidenceof the image display light L on the concavely curved surface producesastigmatism. “Astigmatism” is defined here as non-coincidence of thefocal point of the composite optical system within the meridional planeand the focal point within the sagittal plane. Astigmatism produces ashift between the imaging position of the virtual image 53 in thehorizontal direction (x direction) and the imaging position in thevertical direction (y direction) and results in reduction of the imagingperformance. The term “meridional plane” refers to a plane that includesthe light axis of the composite optical system and the principal ray ofthe image display light L. The yz plane in FIG. 4 represents themeridional plane. Meanwhile, the term “sagittal plane” refers to a planethat includes the light axis of the composite optical system and is aplane perpendicular to the meridional plane. The xz plane in FIG. 4represents the sagittal plane.

FIGS. 5A and 5B schematically show the astigmatism of parallel beamsincident on a partial region 98 on a concavely curved surface 96 andshow views from different points of view. FIG. 5A shows light beamswithin the meridional plane (yz plane) of the concavely curved surface96, and FIG. 5B shows light beams within the sagittal plane (xz plane)of the concavely curved surface 96. As illustrated, the points ofconvergence Fm and Fs of the parallel beams differ between themeridional plane and the sagittal plane. The focal point Fs within thesagittal plane is located farther from the concavely curved surface 96than the focal point Fm within the meridional plane. This is because ofthe fact that, when parallel light beams are caused to be diagonallyincident on a concave mirror, the distance to the position ofconvergence of light (i.e., the focal distance) could change inaccordance with the angle of incidence. Denoting the focal distance ofthe concave mirror by f and the angle of incidence and reflection oflight on the concave mirror by φ, the focal distance of diagonallyincident light is given by f·cos φ. The larger the angle of incidenceand reflection φ, the smaller the focal distance f·cos φ. In otherwords, the focal distance within the meridional plane on which the lightbeam is diagonally incident will be shortened to f·cos φ. Meanwhile, thefocal distance within the sagittal plane will be enlarged to f/cos φ.

In order to reduce the astigmatism As defined as a shift between thefocal point Fm within the meridional plane and the focal point Fs withinthe sagittal plane, the angle of incidence and reflection φ on theconcavely curved surface 96 may be reduced. However, it is not realisticto reduce the angle of incidence and reflection φ of the image displaylight L on the virtual image presentation plate 22 significantly. Thisis addressed in this embodiment by configuring the projection opticalsystem 14 to reduce the astigmatism in the composite optical systemformed by the virtual image presentation plate 22 and the projectionoptical system 14 as a whole. More specifically, mitigation of theoccurrence of double images and reduction of the astigmatism in thecomposite optical system are both achieved by using one or more of thefeatures listed in (1)-(5) below.

(1) Arrange the virtual image presentation plate and the concave mirrorat twisted positions.(2) Arrange projection mirrors having different curvatures in thesagittal direction and in the meridional direction.(3) Insert a plane parallel plate an angle to the light axis.(4) Combine two concave mirrors at mutually twisted positions.(5) Arrange a projection mirror having a surface oppositely curved(having the opposite concavo-convex shape) relative to the virtual imagepresentation plate.

First Embodiment

FIGS. 6A-6C show a configuration of the virtual image display device 10according to the first embodiment in detail and show views fromdifferent points of view. FIG. 6A corresponds to FIG. 1 and shows aconfiguration viewed on the yz plane. FIG. 6B shows a configurationviewed on the xz plane, and FIG. 6C shows a configuration viewed on thexy plane. The embodiment uses the feature (1) above, the concave mirror16 included in the projection optical system 14 is arranged at aposition twisted with respect to the virtual image presentation plate22.

The virtual image display device 10 includes an illumination unit 11, adisplay unit 12, and a projection optical system 14. The projectionoptical system 14 includes a concave mirror 16 and a convex lens 18. Theillumination unit 11, the display unit 12, the convex lens 18, and theconcave mirror 16 are arranged on the light axis extending in the xdirection. The concave mirror 16 reflects, toward the virtual imagepresentation plate 22 and in the y direction, the image display light Lincident in the x direction. The virtual image presentation plate 22reflects, toward the user E and in the z direction, the image displaylight L incident in the y direction.

The virtual image presentation plate 22 is configured such that thefirst surface 23 is concavely curved. The first surface 23 of thevirtual image presentation plate 22 is a first concavely curved surfaceon which the image display light L is incident and reflected diagonally.The virtual image presentation plate 22 is configured to have a uniformthickness t, and the curved surfaces of the first surface 23 and thesecond surface 24 are identically shaped. The virtual image presentationplate 22 is arranged such that the direction (axis A) orthogonal to boththe direction of incidence and the direction of reflection of the imagedisplay light L on the first surface 23 (first concavely curved surface)is the x direction.

The concave mirror 16 is a projection mirror that projects the imagedisplay light L toward the virtual image presentation plate 22. Theconvex lens 18 is provided between the display unit 12 and the concavemirror 16. The concave mirror 16 has a second concavely curved surfaceon which the image display light L is incident and reflected diagonallyand is arranged such that the direction (axis B) orthogonal to both thedirection of incidence and the direction of reflection of the imagedisplay light L on the second concavely curved surface is the zdirection. Therefore, the orientation of the axis B of the concavemirror 16 is orthogonal to the orientation of the axis A of the virtualimage presentation plate 22, and the concave mirror 16 and the virtualimage presentation plate 22 are in a twisted arrangement.

In this embodiment, the virtual image presentation plate 22 and theconcave mirror 16 are in a twisted arrangement so that the astigmatismproduced in the virtual image presentation plate 22 and the astigmatismproduced in the concave mirror 16 occur in the opposite directions.Assuming, for example, that parallel beams are incident on themeridional plane (yz plane) of the virtual image presentation plate 22,the parallel beams are reflected by the virtual image presentation plate22 and the concave mirror 16 and are transmitted through the convex lens18, before being converged on the light axis of the display unit 12 inthe x direction within the xz plane. Therefore, the virtual imagepresentation plate 22 operates, on the light axis of the display unit12, to shorten the focal distance of the light beam within the xz planeand enlarge the focal distance of the light beam within the xy plane.Meanwhile, the concave mirror 16 operates to shorten the focal distanceof the light beam within the xy plane and shorten the focal distance ofthe light beam within the xz plane. Therefore, by combining the virtualimage presentation plate 22 and the concave mirror 16 in a twistedarrangement like this, the astigmatism is reduced as compared with acase where the concave mirror 16 in a twisted arrangement is notprovided.

For reduction of the astigmatism in the composite optical system 20 inwhich the concave mirror 16 and the virtual image presentation plate 22are combined, it will be necessary to ensure that the astigmatism in theconcave mirror 16 and that of the virtual image presentation plate 22are substantially equal. More specifically, this requires ensuring thatthe product f_(a)·cos φ_(a) of the focal distance fa of the concavemirror 16 and the cosine cos φ_(a) of the angle of incidence andreflection of the image display light L on the concave mirror 16 issubstantially equal to the product f_(b)·cos φ_(b) of the focal distancef_(b) of the virtual image presentation plate 22 and the cosine cosφ_(b) of the angle of incidence and reflection of the image displaylight L on the virtual image presentation plate 22. For example,reduction of the imaging performance caused by astigmatism is suitablyprevented by designing the device such that the focal distance f_(a)·cosφ_(a) of the concave mirror 16 within the meridional pane is not lessthan 0.5 times and not more than twice the focal distance f_(b)·cosφ_(b) of the virtual image presentation plate 22 within the meridionalplane.

Further, by providing the display unit 12 at the focal point within themeridional plane of the composite optical system 20 formed by the convexlens 18 and the virtual image presentation plate 22, the occurrence ofdouble images caused by the virtual image presentation plate 22 havingthe two surfaces 23 and 24 is eliminated. The focal point of thecomposite optical system 20 within the meridional plane is the positionof convergence of parallel beams assumed to be incident from the user Etoward the virtual image presentation plate 22 along the meridionalplane (yz plane) of the virtual image presentation plate 22.

According to this embodiment, the occurrence of double images ismitigated without using a virtual presentation image of a customizedspecification such as a wedge glass, by providing the display unit 12 atthe focal point of the composite optical system 20 within the meridionalplane. Further, by providing the concave mirror 16 in a twistedarrangement with respect to the virtual image presentation plate 22, theastigmatism in the composite optical system 20 is reduced and reductionof the imaging performance caused by astigmatism is prevented. This canenhance the visibility of the virtual image 50 presented to the user E.

Second Embodiment

FIG. 7 shows a configuration of a virtual image display device 110according to the second embodiment in detail. This embodiment differsfrom the first embodiment in that the axis A of the virtual imagepresentation plate 22 and the axis B of a concave mirror 116 are alignedin the same direction (x direction), and the concave mirror 116 is notin a twisted arrangement with respect to the virtual image presentationplate 22.

The virtual image display device 110 includes an illumination unit 11, adisplay unit 12, and a projection optical system 114. The projectionoptical system 114 includes a concave mirror 116 and a convex lens 18.The concave mirror 116 is a projection mirror having a concavely curvedsurface on which the image display light L is incident and reflecteddiagonally. The illumination unit 11, the display unit 12, the convexlens 18, and the concave mirror 116 are arranged on the light axisextending in the z direction. The concave mirror 116 reflects the imagedisplay light L incident in the z direction toward the virtual imagepresentation plate 22. The virtual image presentation plate 22 reflectsthe image display light L from the concave mirror 116 toward the user E.

The concave mirror 116 is configured such that the curvature within themeridional plane (first cross section) and the curvature within thesagittal plane (second cross section) are different in order to reducethe astigmatism in the composite optical system 120 formed by theprojection optical system 114 and the virtual image presentation plate22. The meridional plane of the concave mirror 116 is a plane (yz plane)that includes both the direction of incidence and the direction ofreflection of the image display light diagonally incident on the concavemirror 116 and that intersects the concavely curved surface of theconcave mirror 116. Meanwhile, the sagittal plane of the concave mirror116 is a plane orthogonal to the meridional plane and is a plane that isalong the direction (x direction) orthogonal to both the direction ofincidence and the direction of reflection of the image display light andthat intersects the concavely curved surface of the concave mirror 116.The curvature within the meridional plane is related to the focaldistance of parallel beams incident along the meridional plane, i.e.,related to the focal distance within the meridional plane. Meanwhile,the curvature within the sagittal plane is related to the focal distancewithin the sagittal plane.

In this embodiment, the focal point of the composite optical system 120within the meridional plane and the focal point within the sagittalplane are made to coincide by appropriately setting the curvature of theconcave mirror 116 within the meridional plane and the curvature withinthe sagittal plane. For example, the difference between the focaldistance within the meridional plane and the focal distance within thesagittal plane is reduced by configuring the curvature of the concavemirror 116 within the meridional plane to be smaller than the curvaturewithin the sagittal plane. It is preferred to set the specific curvatureof the concave mirror 116 in accordance with the curvature of thevirtual image presentation plate 22 within the meridional plane and thecurvature thereof within the sagittal plane, the angle of incidence andreflection φ_(a) of the image display light L on the virtual imagepresentation plate 22, and the angle of incidence and reflection φ_(b)of the image display light L on the concave mirror 116.

According to this embodiment, the astigmatism in the composite opticalsystem 120 is reduced and reduction of the imaging performance caused byastigmatism is prevented by configuring the curvature of the concavemirror 116 within the meridional plane and the curvature thereof withinthe sagittal plane to be different. Further, the occurrence of doubleimages is inhibited by providing the display unit 12 at the focal pointof the composite optical system 120 within the meridional plane. In thisway, a highly visible virtual image 150 is presented to the user.

It should be noted that this embodiment is applicable to a case wherethe first surface 23 of the virtual image presentation plate 22 is aflat surface. In this case, the astigmatism in the virtual imagepresentation plate 22 is negligible. It therefore suffices to set thecurvature of the concave mirror 116 within the meridional plane and thecurvature thereof within the sagittal plane to have different values soas to reduce the astigmatism in the concave mirror 116.

Third Embodiment

FIG. 8 shows a configuration of a virtual image display device 210according to the third embodiment in detail. This embodiment differsfrom the foregoing embodiments in that a plane parallel plate 219provided at an angle between the display unit 12 and a convex lens 218is added.

The virtual image display device 210 includes an illumination unit 11, adisplay unit 12, and a projection optical system 214. The projectionoptical system 214 includes a concave mirror 216, a convex lens 218, andthe plane parallel plate 219. The illumination unit 11, the display unit12, the plane parallel plate 2149, the convex lens 218, and the concavemirror 216 are arranged on the light axis extending in the z direction.

The plane parallel plate 219 is a transparent member having a uniformthickness and is made of glass or a resin material. The plane parallelplate 219 is provided at an angle to the light path of the projectionoptical system 214 such that the rotational axis C is aligned with thedirection (x direction) orthogonal to the meridional plate (yz plane).Inserting the plane parallel plate 219 at an angle enlarges the focaldistance of a composite optical system 220 formed by the virtual imagepresentation plate 22 and the projection optical system 214. Inparticular, configuring the direction of the rotational axis C of theplane parallel plate 219 to be orthogonal to the meridional planeenlarges the focal distance within the meridional plane as compared withthe focal distance within the sagittal plane. This reduces thedifference between the focal distance within the meridional plane andthe focal distance within the sagittal plane and mitigates theastigmatism as compared with a case where the plane parallel plate 219is not inserted. The amount by which the focal distance increases byproviding the plane parallel plate 219 depends on the angle ofinclination φ_(c) of the plane parallel plate 2219 so that theastigmatism can be adjusted by changing the angle of inclination φ_(c).

According to this embodiment, the astigmatism in the composite opticalsystem 220 is reduced by inserting the plane parallel plate 219.Further, the occurrence of double images is inhibited by providing thedisplay unit 12 at the focal point of the composite optical system 220within the meridional plane. This can enhance the visibility of avirtual image 250.

Fourth Embodiment

FIGS. 9A and 9B show a configuration of a virtual image display device310 according to the fourth embodiment in detail and show views fromdifferent points of view. FIG. 9A corresponds to FIG. 1 and shows aconfiguration viewed on the yz plane, and FIG. 9B shows a configurationviewed on the xz plane. This embodiment differs from the foregoingembodiments in that the projection optical system 314 includes twoconcave mirrors 316 and 318, and the two concave mirrors 316 and 318 arein a twisted arrangement.

The virtual image display device 310 includes an illumination unit 11, adisplay unit 12, and a projection optical system 314. The projectionoptical system 314 includes a first concave mirror 316 and a secondconcave mirror 318. The illumination unit 11, the display unit 12, andthe second concave mirror 318 are arranged on the light axis extendingin the x direction. The second concave mirror 318 is arranged such thatthe direction (axis D) orthogonal to both the direction of incidence andthe direction of reflection of the image display light L is the ydirection and reflects, in the z direction, the image display light Lincident in the x direction. The first concave mirror 316 is arrangedsuch that the direction (axis B) orthogonal to both the direction ofincidence and the direction of reflection of the image display light Lis the x direction and reflects the image display light L incident inthe z direction toward the virtual image presentation plate 22.

Since the first concave mirror 316 and the second concave mirror 318 arein a twisted arrangement according to the embodiment, the astigmatismproduced in the respective concave mirrors is made to occur in theopposite directions. The mechanism to mitigate the astigmatism bycombining two concavely curved surfaces is as described in the firstembodiment. Therefore, this embodiment can also reduce the astigmatismin the composite optical system 320 formed by the virtual imagepresentation plate 22 and the projection optical system 314 and enhancethe imaging performance. Further, the occurrence of double images isinhibited by providing the display unit 12 at the focal point of thecomposite optical system 320 within the meridional plane. This canenhance the visibility of the virtual image 350.

It should be noted that this embodiment is applicable to a case wherethe first surface 23 of the virtual image presentation plate 22 is aflat surface. In this case, the astigmatism in the virtual imagepresentation plate 22 is negligible. It therefore suffices to set thecurvature and the angles of incidence and reflection φ_(b), φ_(d) of thetwo concave mirrors 316, 318 respectively so as to reduce theastigmatism in the combination of the first concave mirror 316 and thesecond concave mirror 318.

In this embodiment, the projection optical system 314 is not providedwith a convex lens, but an additional convex lens may be provided in theprojection optical system 314. For example, an additional convex lensmay be provided between the display unit 12 and the second concavemirror 318, or an additional convex lens may be provided between thesecond concave mirror 318 and the first concave mirror 316.

Fifth Embodiment

FIG. 10 shows a configuration of a virtual image display device 410according to the fifth embodiment in detail. This embodiment differsfrom the foregoing embodiments in that a convex mirror 416 is used tomitigate the astigmatism produced in the virtual image presentationplate 22. In other words, the concavo-convex shapes of the virtual imagepresentation plate 22 and the projection mirror are reversed to producethe astigmatism in the respective curved surfaces in the oppositedirections, thereby reducing the astigmatism in the composite opticalsystem as a whole.

The virtual image display device 410 includes an illumination unit 11, adisplay unit 12, and a projection optical system 414. The projectionoptical system 414 includes a convex mirror 416 and a convex lens 418.The illumination unit 11, the display unit 12, the convex lens 418, andthe convex mirror 416 are arranged on the light axis extending in the zdirection. The convex mirror 416 is a projection mirror having aconvexly curved surface of incidence and reflection of the image displaylight L. The convex mirror 416 is arranged such that the direction (axisB) orthogonal to both the direction of incidence and the direction ofreflection of the image display light L is the x direction and reflectsthe image display light L incident in the z direction toward the virtualimage presentation plate 22.

The virtual image presentation plate 22 is configured such that thefirst surface 23, on which the image display light L is incident, isconcavely curved and is arranged such that the direction (axis A)orthogonal to both the direction of incidence and the direction ofreflection of the image display light L is the x direction. The virtualimage presentation plate 22 reflects the image display light L from theconvex mirror 416 toward the user E.

According to this embodiment, the virtual image presentation plate 22and the convex mirror 416 are arranged to face each other such that therespective curved surfaces have the opposite concavo-convex shapes.Therefore, the astigmatism produced in the respective curved surfaces isreduced or canceled. This reduces the astigmatism in a composite opticalsystem 420 formed by the virtual image presentation plate 22 and theprojection optical system 414 and enhances the imaging performance.Further, the occurrence of double images is inhibited by providing thedisplay unit 12 at the focal point of the composite optical system 420within the meridional plane. Thus, according to this embodiment,astigmatism and doubles images are both reduced, and the visibility of avirtual image 450 presented to the user E is enhanced.

The present invention has been described above with reference to theembodiment but is not limited to the embodiment. Appropriatecombinations or replacements of the features of the illustrated examplesare also encompassed by the present invention.

The embodiment described above shows that the astigmatism in thecomposite optical system is mitigated by using one of the features(1)-(5) described above. In one variation, a plurality of features maybe combined. For example, the feature (2) or the feature (3) may becombined with the feature (1), or the feature (3) or the feature (4) maybe combined with the feature (2). Also, the feature (4) or the feature(5) may be combined with the feature (3).

The embodiment described above shows that the first surface 23 of thevirtual image presentation plate 22 is formed by a concavely curvedsurface or a flat surface. In one variation, the composite opticalsystem may be configured to mitigate the astigmatism of the convexlycurved first surface 23 of the virtual image presentation plate 22. Inthe case of the feature (1), the concave mirror 16 may be replaced by aconvex mirror. In the case of the feature (2), curvature within themeridional plane may be larger than the curvature within the sagittalplane. In the case of the feature (5), the convex mirror 416 may bereplaced by a concave mirror.

The embodiment described above shows that a convex lens is included inthe projection optical system. In one variation, a convex lens may notbe provided in the projection optical system. In the case of theembodiment shown in FIGS. 6A-6C, for example, the convex lens 18 may notbe included in the projection optical system 14, and the display unit 12may be provided at the focal point within the meridional plane of thecomposite optical system 20 not including the convex lens 18. Similarly,the convex lens 18 may not be included in the projection optical system114 of the embodiment shown in FIG. 7, and the convex lens 218 may notbe included in the projection optical system 214 of the embodiment shownin FIG. 8.

One embodiment of the disclosure is summarized below.

(Item 1-1)

A virtual image display device for presenting a virtual image to a uservia a virtual image presentation plate, comprising:

a display unit that generates an image display light; and

a projection optical system that includes a projection mirror thatprojects the image display light toward the virtual image presentationplate such that the image display light is incident and reflected on afirst concavely curved surface of the virtual image presentation platediagonally, wherein

the projection mirror has a second concavely curved surface on which theimage display light is incident and reflected diagonally and is arrangedsuch that a direction orthogonal to both a direction of incidence and adirection of reflection of the image display light on the firstconcavely curved surface and a direction orthogonal to both a directionof incidence and a direction of reflection of the image display light onthe second concavely curved surface are orthogonal to each other, and

the display unit is provided at a focal point of a composite opticalsystem within a meridional plane, the composite optical system beingformed by the virtual image presentation plate and the projectionoptical system, and the focal point of the composite optical systemwithin the meridional plane is a position of convergence of parallelbeams assumed to be incident from the user toward the virtual imagepresentation plate along the meridional plane of the virtual imagepresentation plate.

(Item 1-2)

The virtual image display device according to item 1-1, wherein theprojection mirror is configured such that a curvature in a second crosssection intersecting the second concavely curved surface is larger thana curvature in a first cross section intersecting the second concavelycurved surface, the first cross section is a plane that includes boththe direction of incidence and the direction of reflection of the imagedisplay light incident on the second concavely curved surfacediagonally, and the second cross section is a plane orthogonal to thefirst cross section and is a plane along a direction orthogonal to boththe direction of incidence and the direction of reflection of the imagedisplay light incident on the second concavely curved surfacediagonally.

(Item 1-3)

The virtual image display device according to item 1-1 or item 1-2,wherein the projection optical system further includes a plane parallelplate provided at an angle to a light path of the projection opticalsystem.

(Item 2-1)

A virtual image display device for presenting a virtual image to a uservia a virtual image presentation plate, comprising:

a display unit that generates an image display light; and

a projection optical system that includes a projection mirror thatprojects the image display light toward the virtual image presentationplate, wherein

the projection mirror has a concavely curved surface on which the imagedisplay light is incident and reflected diagonally and is arranged suchthat a curvature in a second cross section intersecting the concavelycurved surface is larger than a curvature in a first cross sectionintersecting the concavely curved surface, the first cross section is aplane that includes both the direction of incidence and the direction ofreflection of the image display light incident on the concavely curvedsurface diagonally, and the second cross section is a plane orthogonalto the first cross section and is a plane along a direction orthogonalto both the direction of incidence and the direction of reflection ofthe image display light incident on the concavely curved surfacediagonally,

the display unit is provided at a focal point of a composite opticalsystem within a meridional plane, the composite optical system beingformed by the virtual image presentation plate and the projectionoptical system, and

the focal point of the composite optical system within the meridionalplane is a position of convergence of parallel beams assumed to beincident from the user toward the virtual image presentation plate alongthe meridional plane of the virtual image presentation plate.

(Item 2-2)

The virtual image display device according to item 2-1, wherein theprojection optical system further includes a plane parallel plateprovided at an angle to a light path of the projection optical system.

(Item 3-1)

A virtual image display device for presenting a virtual image to a uservia a virtual image presentation plate, comprising:

a display unit that generates an image display light; and

a projection optical system that includes a projection mirror thatprojects the image display light toward the virtual image presentationplate, wherein

the projection optical system includes a projection mirror that projectsthe image display light toward the virtual image presentation plate anda plane parallel plate that is provided at an angle to a light path ofthe projection optical system,

the display unit is provided at a focal point of a composite opticalsystem within a meridional plane, the composite optical system beingformed by the virtual image presentation plate and the projectionoptical system, and the focal point of the composite optical systemwithin the meridional plane is a position of convergence of parallelbeams assumed to be incident from the user toward the virtual imagepresentation plate along the meridional plane of the virtual imagepresentation plate.

(Item 4-1)

A virtual image display device for presenting a virtual image to a uservia a virtual image presentation plate, comprising:

a display unit that generates an image display light; and

a projection optical system that projects the image display light towardthe virtual image presentation plate, wherein

the projection optical system includes a first concave mirror thatreflects the image display light toward the virtual image presentationplate and a second concave mirror that reflects the image display lighttoward the first concave mirror,

defining a plane along both a direction of incidence and a direction ofoutput of the image display light on the virtual image presentationplate as a reference plane, the first concave mirror is oriented suchthat the image display light is incident on the first concave mirror ina direction along the reference plane, and the second concave mirror isoriented such that the image display light is incident on the secondconcave mirror in a direction intersecting the reference plane, and

the display unit is provided at a focal point of a composite opticalsystem within a meridional plane, the composite optical system beingformed by the virtual image presentation plate and the projectionoptical system, and the focal point of the composite optical systemwithin the meridional plane is a position of convergence of parallelbeams assumed to be incident from the user toward the virtual imagepresentation plate along the meridional plane of the virtual imagepresentation plate.

(Item 4-2)

The virtual image display device according to item 4-1, wherein a curvedsurface of at least one of the first concave mirror and the secondconcave mirror is configured such that a curvature in a second crosssection intersecting the curved surface is larger than a curvature in afirst cross section intersecting the curved surface, the first crosssection is a plane that includes both the direction of incidence and thedirection of reflection of the image display light incident on thecurved surface diagonally, and the second cross section is a planeorthogonal to the first cross section and is a plane along a directionorthogonal to both the direction of incidence and the direction ofreflection of the image display light incident on the curved surfacediagonally.

(Item 4-3)

The virtual image display device according to item 4-2, wherein theprojection optical system further includes a plane parallel plateprovided at an angle to a light path of the projection optical system.

(Item 5-1)

A virtual image display device for presenting a virtual image to a uservia a virtual image presentation plate, comprising:

a display unit that generates an image display light; and

a projection optical system that includes a projection mirror thatprojects the image display light toward the virtual image presentationplate such that the image display light is incident and reflected on aconcavely curved surface of the virtual image presentation platediagonally, wherein

the projection mirror has a convexly curved surface on which the imagedisplay light is incident and reflected diagonally and is arranged suchthat a direction orthogonal to both a direction of incidence and adirection of reflection of the image display light on the concavelycurved surface and a direction orthogonal to both a direction ofincidence and a direction of reflection of the image display light onthe convexly curved surface are parallel to each other, and

the display unit is provided at a focal point of a composite opticalsystem within a meridional plane, the composite optical system beingformed by the virtual image presentation plate and the projectionoptical system, and the focal point of the composite optical systemwithin the meridional plane is a position of convergence of parallelbeams assumed to be incident from the user toward the virtual imagepresentation plate along the meridional plane of the virtual imagepresentation plate.

(Item 5-2)

The virtual image display device according to item 5-1, wherein theprojection optical system further includes a plane parallel plateprovided at an angle to a light path of the projection optical system.

It should be understood that the invention is not limited to theabove-described embodiment but may be modified into various forms on thebasis of the spirit of the invention. Additionally, the modificationsare included in the scope of the invention.

What is claimed is:
 1. A virtual image display device for presenting a virtual image to a user via a virtual image presentation plate, comprising: a display unit that generates an image display light; and a projection optical system that includes a projection mirror that projects the image display light toward the virtual image presentation plate such that the image display light is incident and reflected on a concavely curved surface of the virtual image presentation plate diagonally, wherein the projection mirror has a convexly curved surface on which the image display light is incident and reflected diagonally and is arranged such that a direction orthogonal to both a direction of incidence and a direction of reflection of the image display light on the concavely curved surface and a direction orthogonal to both a direction of incidence and a direction of reflection of the image display light on the convexly curved surface are parallel to each other, and the display unit is provided at a focal point of a composite optical system within a meridional plane, the composite optical system being formed by the virtual image presentation plate and the projection optical system, and the focal point of the composite optical system within the meridional plane is a position of convergence of parallel beams assumed to be incident from the user toward the virtual image presentation plate along the meridional plane of the virtual image presentation plate.
 2. The virtual image display device according to claim 1, wherein the projection optical system further includes a plane parallel plate provided at an angle to a light path of the projection optical system.
 3. The virtual image display device according to claim 1, wherein the virtual image presentation plate is a windshield provided in a vehicle and having a uniform thickness. 